Photoacid generation has become valuable in the fields of photoresists and cationic polymerization. Cationic photopolymerization has developed into an excellent alternative to free-radical photopolymerization for applications that can take advantage of the high speed, low temperature, and environmental friendliness of radiation curing technology. In contrast with radiation curing processes initiated by free radicals, cationic photopolymerization processes are not inhibited by oxygen, and by employing monomers and oligomers such as epoxides and oxetanes that undergo rapid cationic ring opening polymerization (CROP), shrinkage resulting from polymerization can be dramatically reduced. Since the onium salt photoacid generators (PAGs) that are commonly used to initiate cationic photopolymerization are typically sensitive only to ultraviolet light when irradiated directly, photosensitizer dyes are used in conjunction with the PAGs to enable photoinitiated acid generation and cationic photopolymerization at longer wavelengths in the near ultraviolet and visible spectral regions.
The ability to employ cationic photopolymerization with visible light combined with the attributes of high photosensitivity and low shrinkage in solventless media at ambient temperatures make this technique attractive for applications such as holographic recording. Since this method of recording uses the entire thickness of the medium to store information, the areal data density increases as the medium thickness increases. Increasing the thickness of the media, however, results in an increase in the absorption of the medium, thereby increasing the optical path length of the medium. In the context of a polymerizable medium, lowering the exposure fluence required to initiate sufficient polymerization to reach the entanglement molecular weight and increasing the polymerization kinetics of the medium often requires increasing the concentration of both the photoacid generator and the photosensitizer dye. Increasing the concentration of the photosensitizer dye, however, results in an increase in the absorption of the medium, thereby increasing the optical path length of the medium. The absorbance of the medium can cause undesirable tradeoffs such as non-uniform polymerization throughout the volume of a polymerizable medium, impaired fidelity of holograms recorded in such media and a diminished increase in the dynamic range of a holographic recording medium as a function of increasing the medium's thickness.
The transmittance of the light incident upon the medium, T, decreases with increased absorbance of the medium in accordance with the well known Beer-Lambert law expressed as
                    A        =                                            log              10                        ⁢                          (                                                I                  o                                I                            )                                =                      ɛ            ⁢                                                  ⁢            cl                                              (        1        )            where A is absorbance of the sample medium, and is also referred to as the optical density of the medium, I0 is the intensity of the incident light in units of quanta per second, I is the intensity of the light transmitted through the sample medium in units of quanta per second, c is the concentration of the absorbing species in units of mol liter−1, ε is the molar absorptivitiy in units of liters mol−1 cm−1 and is also referred to as the molecular extinction coefficient, and l is the thickness of the medium in units of cm. Consequently, the amount of light incident upon the sample medium that transmits through the medium, known as the transmittance of the medium, is expressed as
                              T          =                      I                          I              o                                      ⁢                                  ⁢                  and          ⁢                                          ⁢          where                ⁢                                  ⁢                  A          =                                    log              10                        ⁢                          T              .                                                          (        2        )            
The decrease in light intensity with depth into the medium from the front surface of the medium leads to non-uniformity of the extent of polymerization that occurs within the medium, so that less polymerization occurs depthwise in the interior of a medium as compared to at or near the front surface that is exposed to the incident light. Under ideal circumstances, the amount of light penetrating into a medium would be capable of initiating an identical number of polymerization events at all depths and thus the extent of polymerization would be uniform throughout the depth of the medium. In reality this cannot occur in a medium that exhibits reasonable sensitized polymerization kinetics and thus the degree of chemical segregation, concomitant with the extent of polymerization, is nonuniform through the depth of the recording medium. The degree of this nonuniformity can be significant if high absorbance is needed to achieve good recording sensitivity or if increases in media thickness are required to establish a roadmap for increased capacity of information stored in a set form factor such as a disk or card.
The characteristic level of absorbance of a sensitized medium is crucial in applications such as holographic recording media, where uniformity of the refractive index modulation of each hologram, which develops from chemical segregation that is induced by polymerization reactions, is critical. In particular, angle multiplexing methods of various types are used to record holograms in co-locational or substantially overlapped areas in order to achieve high areal density. Such methods typically result in formation of holograms that exhibit diffraction efficiency less than about 0.05% and which are required to exhibit both good angular selectivity characteristics and good image quality. Typically, a first hologram is recorded and then a second hologram is recorded using a reference beam angle where most desirably the first hologram has a first minimum of intensity (the “null” or minimum of the Bragg selectivity curve). Subsequent holograms recorded in substantially the same storage location are similarly recorded most desirably at the first such minimum of intensity of the hologram that is recorded with the most similar reference beam angle. If the Bragg selectivity curve, however, exhibits increased intensity at the angle of the expected first minimum, and thus a poorly defined first minimum exists which, by way of example, is commonly observed as a shoulder of the main Bragg peak, then the multiplexed holograms must instead be recorded at the second minimum or “null” of the Bragg selectivity curve. The deviation from an ideal sinc2 Bragg selectivity profile, that in accordance with coupled wave analysis (see Kolgenik, Bell Syst. Tech. J. 48: 2909, (1969)) represents the theoretical dependence of angle versus intensity for the reconstruction of the holograms, is reduced by a factor of four at t the second “null” and thus the overall crosstalk noise is also reduced by factor of four (see Waldman et al., J. Imag. Sci. Tech., Vol 41, No. 5, pp 508-513 (1997)). Multiplexing holograms at angles corresponding to the second such minimum, however, lowers the areal density that can be achieved for a particular thickness of media by a factor of two from what would most desirably be achieved.
One solution to this problem of cross talk, and to the broader problem of having non-uniform polymerization throughout the volume of a polymerizable medium, is to reduce the extinction coefficient of a photosensitizer dye. The effects of lowering the extinction coefficient or required concentration of a dye are most apparent when it is desirable to use a thicker polymerizable medium (e.g., to hold more information per unit surface area) or when a hologram must have high fidelity. At concentrations that generate both a useful amount of polymerization and high recording sensitivity, presently available photosensitizer dyes are limited to use in polymerizable media with thicknesses of about 300 micrometers or less. This deficiency necessitates the invention of new sensitizer dyes tailored so as to have lower extinction coefficient at the wavelength of interest, thereby permitting its use in higher concentration while maintaining lower overall absorbance. Sufficient dye is then available such that the maximum state of polymerization is limited only by monomer mobility rather than by the number of dye molecules, and exposure non-uniformity is minimized.
Presently available photosensitizer dyes such as rubrene and 5,12-bis(phenylethynyl)naphthacene (BPEN) have extinction coefficients at specific wavelengths of visible light, especially light in the 500-550 nm region (e.g., from commercially-available lasers such as an argon ion laser or frequency-doubled Nd:YAG laser) that result in reduced performance when used in relatively thick holographic recording media having thicknesses greater than about 300 micrometers.
It is therefore desirable to develop photosensitizer dyes with reduced extinction coefficients at selected visible wavelengths, especially in the 500-550 nm region, such that the absorbance of a sufficient quantity of sensitizer dye (e.g., to achieve polymerization) will be decreased to permit light penetration into a polymerizable medium and to preserve acceptable uniformity of polymerization within the polymerizable medium. The recording media is made sensitive to actinic radiation of a desired energy level (wavelength) by the incorporation of a sensitizer dye. The normal polymerization procedure is to irradiate the photopolymer with photons which will then begin the polymerization process. The reaction sequence associated with this process is complex. A simplified, but reasonably good model is as follows: the dye is first excited by photons and then the excited dye transfers energy to the initiator to provide an excited initiator or the excited dye reacts with the initiator via a oxidation-reduction process to form an initiative species. In either case the initiative species or excited initiator then combines with a monomer, which begins a chain reaction with additional monomers to result in polymerization. Photosensitizer dyes should also undergo efficient electron transfer reactions and preferably bleach completely at the relevant wavelength when exposed to visible light in the presence of an onium salt (e.g., a wavelength corresponding to a laser). In addition, photosensitizer dyes should have adequate solubility, especially in cationic polymerization media, and should not inhibit cationic processes (e.g., should be sufficiently non-basic).